In efforts to realize a ultrahigh-speed, broadband optical communications network, high expectations are placed on an optical modulator (an external modulator) of external modulation system.
Particularly, development of a Mach-Zehnder optical modulator using LiNbO3 (lithium niobate; LN) that exhibits a superior high-speed modulation characteristic and a dispersion-resistant characteristic is pursued to enable long-distance transmission of an optical signal.
As shown in FIG. 10, an RZ optical modulator 100 comprising two Mach-Zehnder optical modulators 101, 102 connected in tandem has recently been put forth as an optical modulator capable of generating an RZ (Return to Zero) signal. This RZ optical modulator 100 can produce an RZ-modulated optical signal by causing an electric signal of a clock (a clock signal) to enter one Mach-Zehnder optical modulator 101 and causing an electric signal of NRZ (Non Return to Zero) data (a data signal) to enter the remaining Mach-Zehnder optical modulator 102.
However, in such an RZ optical modulator 100, the two Mach-Zehnder optical modulators 101, 102 are arranged in series on a substrate (in the longitudinal direction of the substrate). Therefore, when compared with, e.g., an NRZ optical modulator which generates an NRZ signal by inputting an NRZ data signal to one Mach-Zehnder optical modulator, the length of a chip becomes about double.
Therefore, the length of the chip is made as short as possible, to thus render the RZ optical modulator 100 compact. A technique for embodying a compact optical modulator by shortening a chip length is described in, e.g., Japanese Patent Laid-Open NO. HEI 5-232417 and Japanese Patent Laid-Open NO. HEI 11-237517.
Meanwhile, in order to realize a compact RZ optical modulator, for instance, mere shortening of a chip length is conceivable. However, the working length [the length of a portion of the optical modulator where an electric signal (the clock signal or the data signal) acts on the light propagating through the optical waveguide; corresponding to the length of a signal electrode] is limited and shortened by the chip size. Hence, realization of a low driving voltage becomes difficult. Namely, if the working length of the Mach-Zehnder optical modulator is usually made long, the driving voltage can be reduced. However, if the chip length is simply shortened to realize a compact RZ optical modulator, the working length will also become shorter. Hence, even when the RZ optical modulator can have been made compact, difficulty is encountered in realizing a low drive voltage.